#include<bits/stdc++.h>
using namespace std;
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
const int N=2e5+10;
#define INF 0x3f3f3f3f;
typedef long long int ll;
#define close(); std::ios::sync_with_stdio(false);cin.tie(0),cout.tie(0);
//----------------------------------------------------------------------------//
ll x, y, k, n, t,t2;
void solve()
{
    cin >> x >> y >> k >> n >> t;

    ll ans2=0,sd2=0,i=0;
    while(n>0)
    {
        ans2+=x*n; 
        sd2=sd2+n;
        n--;i++;
        if(sd2>=k)
        {
            x+=(sd2/k)*y;
            sd2=sd2%k;
        }
        if(ans2>=t)
        {
            cout<<i<<'\n';
            return;
        }

    }
    cout<<-1<<'\n';   
}

int main()
{
    //close();
    //int T; cin>>T;
    //while (T--) solve();
    solve();
    return 0;
}
